In projectors that use reflecting-type spatial modulating elements for the three primary colors red, green and blue, prisms for guiding the illuminating light and prisms for combining colors are disposed between the projecting lens and the spatial modulating elements. Because of this, the projecting lens requires a long back focus. Since spectral characteristics of prisms for combining colors are dependent on the incident angle, it is necessary to have an optical system in which the pupil on the shorter side conjugate distance is a sufficient distance from the spatial modulating elements, that is to say, it is necessary to have telecentricity.
For a forward convex group four-group zoom lens in which the long back focus and telecentricity do not vary with zoom, there is the zoom lens proposed in Patent Reference 1 below, for example. Furthermore, as a forward convex group three-group zoom lens, there is the zoom lens that is proposed in Patent Reference 2 below, for example. As a forward concave group four-group zoom lens, there is the zoom lens proposed in Patent Reference 3 below, for example.
Furthermore, as a telecentric wide angle lens, there is the wide angle lens proposed in Patent Reference 4 below.
There is also a demand for projectors in which the projection distance from the screen to the projector is short and that can be used in a small space, and there is also a demand for wide angle lenses that can be used over a short projection distance for projecting lenses.
Furthermore, in the case of wide angle lenses, the manner in which distortions are corrected is very important. Aspheric surfaces have a great ability to correct distortions, and can reduce the outside diameter of the lens and the number of constituent lenses. As a telecentric wide angle lens in which aspheric surfaces are used, there is the wide angle lens proposed in Patent Reference 5, for example.
Furthermore, methods are proposed for obtaining a bright image by combining the image from two projectors onto a screen, and for obtaining an image having a large aspect ratio by lining up two projected screens next to each other.
However, in such projection methods in which two projectors or two screens are used, it is necessary that the corresponding pixels are projected onto the same position by two projectors. If used under these conditions, distortions that were not conventionally a problem become a significant issue.
That is to say, the position of the corresponding pixels that are projected from the two projectors onto a screen shifts with distortions of conventional projection lenses, and there is a considerable loss of resolution. Thus, it is necessary that the distortion of the projecting lens is sufficiently small, however the zoom lens proposed in the above noted Patent Reference 1 has a large distortion value of about −2% at the wide angle end and about +0.3% at the telephoto end.
Furthermore, in addition to the distortions being sufficiently small, it is desirable to have a compact projecting lens that also has a long back focus. However in the zoom lens proposed in the above-noted Patent Reference 2, in addition to the fact that the back focus is insufficient, the distortion is as large at about −2% at the wide angle end and about −1% at the telephoto end and the overall length of the lens is about 11 times the focal length at the wide angle end, thus posing problems for miniaturization.
Furthermore, in the case of the zoom lens proposed in the above-noted Patent Reference 3, the distortion is as large at about −2.7% at the wide angle end and about −1.2% at the telephoto end, the F number is dim at about 3.5 and brightness cannot be ensured.
Next, in the case of the wide angle lens proposed in the above-noted Patent Reference 4, there is insufficient back focus for use as a projecting lens for a projector that uses reflecting-type spatial modulating elements.
Furthermore, in the case of the wide angle lens in which aspheric surfaces are used, proposed in the above-noted Patent Reference 5, there is insufficient correction of axial chromatic aberration and chromatic coma (a condition in which there is no coma aberration with respect to the standard wavelength, but there is downward coma aberration with a red color of 620 nm, and upward coma aberration with a blue color of 460 nm). This is because the aspheric surface has no ability to correct colors.
Accordingly, many wide angle lenses for projectors are used in a rear aspect, and these also may be combined with a backing mirror and used as a single unit. In this case, the lens is projected onto an approximately 178 cm diagonally dimensioned screen, and therefore the lens requires a capability at close distances.
However, the performance of wide angle lenses fluctuates greatly with projecting distance. In particular, as noted above, wide angle lenses for projectors require a long back focus, and the lenses are arranged in a sequence of concave to convex from the side with the longer conjugate distance. This is what is known as a reverse telephoto-type (retro focus-type). In this configuration, asymmetry of the lens arrangement with respect to the aperture stop increases, and change of performance with respect to the change of projection distance becomes larger. On the other hand, in the case of a wide angle lens that is symmetrical with respect to the aperture stop, even if the height of the light beams that pass through the lens changes, there is little change in the performance of the lens because the lens is operated so as to compensate for the aberration around the aperture stop.
That is to say, for a retro focus-type lens such as is described above, since the asymmetry of the lens arrangement with respect to the aperture stop is large, if there is a change in the height of the light beams that pass through the lens due to a change in the projection distance, then the aberrations are not cancelled out and there is a change in the performance.
Consequently, it is a significant problem to ensure the performance of wide angle lenses for projectors, with changes in projection distance when projecting onto screens having a size of 782 to 178 cm, for example.
[Patent Reference 1]
JP H10-161027A
[Patent Reference 2]
JP 2001-215411A
[Patent Reference 3]
JP 2002-131639A
[Patent Reference 4]
JP H11-109227A
[Patent Reference 5]
JP 2002-131636A